1. Field of the Invention
The present invention relates in general to an organic-optic recording tape for writing, reading, erasing and rewriting information by employing laser beams and, more particularly, to an organic-optic recording tape of higher capacity and higher packing density of data, suitable for used in a recording medium for several kinds of information including image information.
2. Description of the Prior Art
In recent, the computer industry has generated an enormous and continually increasing demand for data storage associated with highly informationized society. This demand has necessarily encouraged the continued development of recording media of higher capacity, higher packing density of data and rapid access time. In order to achieve such recording media, there has been proposed a magnetic medium which permits the information to be recorded along a direction of magnetization of its magnetic recording layer, i.e., the longitudinal direction of the magnetic recording layer. Such a magnetic medium is practically used for, for example, video tapes, audio tapes, floppy discs and etc.
The demand for data storage has also encouraged the development of a recording medium having not only higher capacity but also desirable compact size and, as a result, the development of a magneto-optic recording medium. In this magneto-optic recording medium, the magneto-optic recording layer is magnetically polarized in vertical direction on the recording layer as opposed to the aforementioned magnetic medium. In addition, this magneto-optic recording medium has intensive coercive force, maintaining the direction of magnetization, of about 5 to 10 times that of the above magnetic medium and this makes it difficult to change the direction of magnetization by outside magnetic field. In this respect, in order to change the direction of magnetization of the magneto-optic layer, it is required to focus a laser beam on the recording layer such that the laser beam has a very small focused spot of 1 .mu.m or less in diameter and heats the recording layer locally to a temperature not less than the Curie temperature of the layer. At this state, outside magnetic field is employed so as to change the direction of magnetization. Hence, it is possible to record the information on the recording layer along the changed direction of magnetization. In accordance with such a magneto-optic recording, information recording unit on the recording layer is micronized to 1 .mu.m or less in diameter and, in this respect, provides higher packing density of data of about 10 to 1000 times that of the conventional magnetic recording. In addition, this magneto-optic recording allows the information to be read by employing non-contact reading, thereby facilitating data preservation and lengthening the using life of the recording medium.
However, a disadvantage of this magneto-optic recording is resided in that its recording medium necessarily uses magnetic materials of heavy metals and employs physical vapor deposition process, such as vacuum evaporation or sputtering, in its preparation and, as a result, has serious difficulty in the preparation process. In an effort to provide recording media overcoming the above disadvantage of the magneto-optic recording medium, there has been proposed organic-optic recording media. These organic-optic recording media are generally classified into two types in consideration of whether they are erasable and rewritable. Otherwise stated, one type is so-called WORM (Write Once Read Many) type organic-optic recording media which permit the information to be wrote once and read many times while the other type is so-called rewritable type organic-optic recording media which permit the information to be erased and rewritten if necessary.
Each of the WORM type recording media includes a substrate coated with a reflection layer and a recording layer deposited on the substrate. In order to provide the recording layer, laser absorbing colorants mixed with a high-molecular resin is deposited on the substrate. In addition, A preservation layer is coated on the recording layer. The recording of data on such a WORM type recording medium is effected by directing a laser beam to the recording layer in order to form a small focused laser spot of 1 .mu.m or less in diameter. Hence, the laser absorbing colorants of the recording layer absorb the laser beam and generate heat which causes the high-molecular resin as well as the laser absorbing colorants to be melted or decomposed in order to form a pit. As a result, the information can be recorded on the recording medium. The reading of data (logic 1 or logic 0) is effected by reflectivity difference between the pits and the normal part of the medium. In this WORM recording medium, it is impossible to erase and rewrite the information since the recording part of the medium, i.e., the pits, has the form of decomposition of the high-molecular resin.
Meanwhile, the recording layer of the rewritable type organic-optic recording medium is prepared by dispersing colorants in an organic high-molecular resin. The recording of the information on this type of recording medium is effected by directing the laser beam to the recording layer in order to thermally expand the organic high-molecular resin and to form a bump on the recording layer and, thereafter, by maintaining the bump as it was. This rewritable type recording medium using organic colorants has been disclosed, for example, in U.S. Pat. No. 4,719,615 to Feyrer el al. (Optical Data Inc.) which will be described hereinbelow in conjunction with FIGS. 1A to 1C.
As shown in these drawings, this recording medium is an optical data storage disc having three-layer construction. Otherwise stated, this optical disc includes a substrate 1 and a dual layer deposited on the substrate 1, the dual layer having a bottom layer or a recording layer 2 of one material and a top layer or an rewritable layer 3 of another material.
The substrate 1 is made of a transparent resin, such as polycarbonate or polymethacrylate. In addition, the recording layer 2 is made of an elastic high-molecular resin and laser absorbing colorants, while the rewritable layer 3 is made of a thermoplastic high-molecular resin and laser absorbing colorants. The recording of the information on the recording layer 2 is effected by directing a recording laser beam to the layer 2 in order to cause the laser absorbing colorants to absorb the laser beam and generate heat for thermally expanding the high-molecular resin. The thermal expansion of the resin causes irreversible deformation of the rewritable layer 3 and, as a result, forms a bump. After the laser scanning, the rewritable layer 3 rapidly cools down as it was. As a result, the bonding force between the two layers 2 and 3 overcomes restoring force of the elastic resin of the recording layer 2. In this respect, this recording layer 2 is prevented from restoring its initial plane shape due to the bonding force and the bump shape of the rewritable layer 3, thereby maintaining the bump Bu as it was as represented in FIG. 1B. The reading of the information is effected by directing a reading laser beam, having less intensity than that of the recording laser beam, on the optical disc. This recording laser scanning results in reflectivity difference between the bumps and the normal part of the recording layer 2 and this allows the information to be read from the layer 2, for example, in such a manner that the information of logic 1 is read from the bump BU while the information of logic 0 is read from the normal part. In order to erase the information, an erasing laser beam is focused on the rewritable layer 3 and absorbed by the colorants of the layer 3. When these colorants absorb the laser beam, they generate heat for heating the rewritable layer 3 to a temperature not less than a glass transition temperature Tg of the layer 3. The resisting force against the restoring force of the elastic high-molecular resin of the recording layer 2 is, therefore, reduced and, as a result, the recording layer 2 restores its initial plane shape as represented in FIG. 1C.
Examples of the laser beam absorbing colorants, that can be used in the recording and rewritable layers 2 and 3 are phthalocyanines, carbon blacks, azos (monoazo and diazo), anthroquinones, nigrosenes and xanthenes. In order to carry out the optical writing, reading, erasing and rewriting of data on the optical data storage disc, it is required to provide an electro-optical system including a pair of variable-intensity lasers for outputting individual laser beams, a pair of collimating lenses for collimating the laser beams, a reflection mirror, a beam splitter, an objective lens and etc.
However, this known optical recording technology such as disclosed in the above U.S. Patent, while being applicable to the organic-optic recording which is associated with this invention, nevertheless has a disadvantage in that it requires the pair of variable-intensity lasers for recording and erasing of data and, as a result, necessarily has a complex construction which resists to the recent trend of compactness of the practical electro-optical system. Another disadvantage of the above known technology is resided in that the data storage capacity is restricted due to the limited recording area of the data storage disc. In addition, the preparation process for the disc according to this known technology requires two coating steps and two drying steps for providing the dual layer having the recording and rewritable layers. In coating the rewritable layer on the recording layer, it is required to satisfy properties, running counter to each other, for providing desired bonding force between the two layers with no damage of the recording layer and, furthermore, to disperse different colorants in the recording and rewritable layers. In this respect, the preparation process for the disc is very complex.